The present invention generally relates to a graphics processing apparatus, and more particularly to a graphics processing apparatus in which an antialiasing process is carried out in order to smooth edges of an output image by eliminating irregularities or aliases at edge portions of the output image.
In computer graphics systems, an antialiasing technique is used to smooth edges of an output image when the image is output to a cathode ray tube (CRT) display device or a printer connected to a personal computer, so that the output image having smooth edges can be seen on the CRT display screen or the printed sheet. When an antialiasing process is performed with respect to an image to be processed, a luminance or intensity of each dot at edges of the image is modulated and staircase-like irregularities (or, aliases) at the edge portions of the image as shown in FIG. 1A are eliminated so as to create visually smooth edge portions of the image as shown in FIG. 1B. In the prior art, there are several kinds of antialiasing techniques for performing an antialiasing process with respect to an image to be processed. The antialiasing techniques include the averaging technique, the filtering technique, the prefiltering technique and so on. These antialiasing techniques are described in detail in the copending U.S. patent application Ser. No. 680,697 filed on Apr. 4, 1991 having been assigned to the assignee which is the same as that of the present invention. The disclosure of this application is hereby incorporated in the present specification explicitly by reference.
With the recent development of a desktop publishing (DTP) system utilizing a personal computer, it is possible to print out a computer graphic image described by vector data. One of such DTP systems is the PostScript system of Adobe Co. The PostScript is a kind of programming language known as the Page Descriptor Language (PDL). This programming language is used to describe a picture or graphics image of a page and define the arrangement of image data of that page, including the style and format of the text region and the graphics region within the page. In this DTP system, vector font data is used instead of a character font used in the ordinary image processing system. The image output by the above described DTP system can have better quality than the quality of an image output by a conventional wordprocessor using dot matrix format. Also, the DTP system has an advantageous feature in that a composite image in which a text image and a graphics image coexist can be easily and smoothly processed and output to a CRT display or a laser printer.
When a laser printer for outputting an image with a relatively low resolution of 200 dpi to 400 dpi is used, there is a problem in that the effects of aliasing appear at edges of a graphics image output by the laser printer. It is desirable to eliminate the aliases from the edges of such an image, and to produce a high quality image by using an improved antialiasing process. In the cases of the existing image processing systems, the antialiasing process, which is time-consuming and complicated, must be performed in order to determine an approximate area factor or a luminance level per pixel in the image to be processed. It is difficult for the above mentioned apparatus to smoothly increase the speeds of the antialiasing and printing processes at a low cost. Especially when the prefiltering technique is used, it is necessary to perform many computation steps, and the computation steps highly depend on the process of producing a number of subpixels from a pixel. Thus, it is difficult to carry out a fast, efficient graphics processing by means of the existing graphics processing systems.
In addition, Japanese Laid-Open Patent Publication No. 2-112966 discloses an apparatus for performing an antialiasing process so that a high quality image is output to a display unit. In this apparatus, after bi-level image data described in dot matrix format is received, the bi-level image data is grouped into a plurality of small blocks, a pattern matching is performed with respect to each block, and an antialiasing process is performed with respect to each pixel of a target block so as to improve the picture quality of the image. However, a luminance level for a pixel of the image is selected from among intermittent luminance levels, and it is difficult to prevent the luminance level of the image from being discontinuous with respect to some successive pixels of the image. Also, the results of the antialiasing process are not effective.